The present invention relates to a copier, printer, facsimile, and to a control of a fixing apparatus used for an electro-photographic type image forming apparatus such as a complex machine provided with these functions, and particularly, to a fixing apparatus for heating, pressuring and fixing a toner image formed in a recording material in a nip portion formed between a fixing roller and an endless fixing belt, and to an image forming apparatus provided with the fixing apparatus by which a wrinkle is not generated even when an envelope is passed through.
In the electro photographic type image forming apparatus such as the complex machine provided with the copier, printer, facsimile, and these functions, a latent image corresponding to a document is formed on a photoreceptor drum and visualized when the toner is given to this latent image, and this visualized toner image is transferred onto the recording material, and after that, the toner image on the recording material is fixed and sheet-delivered.
Further, when a color image is formed, the latent image of Y, M, C, K corresponding to a document color is formed on the 4 photoreceptor drums, and after the visualized 4 color toner images are primarily transferred onto the endless inter mediate transfer belt, secondarily transferred onto the recording material and the toner image transferred onto the recording material is fixed and sheet-delivered.
As the fixing apparatus for fixing the toner image in such a manner, there is the fixing apparatus of the heat roller fixing system by which while the recording material onto which the toner image is transferred is nipped and conveyed by a fixing roller in which a heating means such as a halogen lamp is housed, and a nip portion formed by a pressure roller for pressing the fixing roller, it is heated and pressed, and because, in such a fixing apparatus, the structure is simple, it is widely used.
Hereupon, in such a fixing apparatus, in order to intend the high speeding-up, it is necessary that enough heat amount is supplied to the toner and the recording material, therefore, it is necessary that the nip width is widened. In order to widen the nip-width, it is considered to increase the pressing force which the pressing roller presses the fixing roller, the thickness of the elastic layer formed of silicon rubber in the pressing roller, or the diameter of two rollers.
However, when the pressing force or the thickness of the elastic layer is increased, there is a case where the nip width in the axial direction becomes un-uniform, and there is a possibility that uneven fixing, or a wrinkle of recording material is generated. Further, when the diameter of the roller is increased, there is a problem that not only a case where the size of the fixing apparatus is increased, but also a warming-up time is extended.
In order to solve this problem, the fixing apparatus which is provided with the fixing roller which has the elastic layer formed of silicon rubber, and the heating means such as a halogen lamp is housed in the center, and rotated, the endless fixing belt which is driven by the fixing roller and rotated, and an elastic pressing member arranged inner peripheral surface side of the fixing belt, and by the elastic pressing member, which pressure-contacts the fixing belt with the fixing roller, is disclosed in Tokkai No. 2005-173441.
According to this fixing apparatus, the fixing belt which is in pressure contacted with the fixing roller by the elastic pressing member is elastic deformed, the wide nip portion is formed between the fixing roller and the fixing belt. Accordingly, it can correspond to the high speeding-up and the size of the fixing apparatus is not increased. Further, because the heat capacity of the fixing belt is small, the warming-up time is reduced, and the energy saving is attained.
Herein, as the recording material to be passed through, excepting the sheet which is only one sheet such as the plain paper, there is a case where an envelope is used. When the envelope is used, because both ends of the sheet material are fixed, and 2 sheets are superimposed between them, when the envelope passes through the circular arc-likely formed nip portion, the sheet material positioned on the fixing roller side is positioned in the inside of the circular arc, the sheet material positioned on the fixing belt side is positioned on the outside of the circular arc, and the conveyance speed of the latter one is faster than the former one, and because the conveyance speed of the both are different, the wrinkle is easily generated in the latter one.
For this reason, in the fixing apparatus in the same manner as the fixing apparatus written in Japanese Patent Publication Open to Public Inspection No. 2005-173441, in the case of envelope mode in which the envelope is passed through, the fixing apparatus in which the contact-pressing force of the elastic pressing member is reduced, and the generation of wrinkle is prevented, is disclosed in Japanese Patent Publication Open to Public Inspection No. 10-228200.
However, in the fixing apparatus written in Japanese Patent Publication Open to Public Inspection No. 10-228200, when the envelope is passed through, although the generation of the wrinkle can be prevented, when the contact-pressing force of the elastic pressing member is decreased, the fixing property is deteriorated. In order to prevent the deterioration of the fixing property, it may be allowable when the conveyance speed of the envelope is decreased, however, the number of output sheets per unit time is decreased, and a problem that the productivity is decreased, is generated.